1. Field of the Invention
The invention relates to an electric heating device for heating air, which in particular is suitable for use as supplemental electric heating in motor vehicles.
2. Description of the Related
For use in motor vehicles, particularly in such having motors optimised for the least possible consumption, electric heating devices are used for heating the interior and/or the motor. Supplemental electric heating is in particular needed after starting the motor as long as the combustion engine does not provide sufficient thermal energy. Combustion engines optimised for the least possible consumption basically even require the use of supplemental electric heating.
The use of such heating devices is, however, not limited to the field of motor vehicles; they are also suitable for a number of other uses, for instance in the field of home installations (room air conditioning), industrial plants, etc.
From EP-A2-0 902 311 an electric heating device for motor vehicles is known. The heating device described comprises several heating elements joined together in a heater block. The heater block, together with a control unit for controlling the heating elements, is held in a common frame. The control unit thus constitutes a structural unit with the heater block held in the frame. The control unit comprises a power electronics system with electronic switches each of which is provided with a cooling body. The control unit is arranged such that a portion of the air current to be heated is blown onto the control unit, in particular the cooling elements, for cooling the electronic switches.
The electronic switches, in particular in the form of power transistors, which control the power fed to the heating elements, are directly mounted on a printed circuit board on one side. To remove the dissipation heat generated by the power transistors a cooling element or cooling plate is provided on the opposite side of the printed circuit board. In order to improve the heat transfer between the transistor and the cooling element through the printed circuit board, the printed circuit board is provided in the area between the transistor and the cooling element with a number of feed-throughs. A disadvantage with this well known approach is that the heat conductivity of these feed-throughs does not suffice to remove the dissipation heat. Although these feed-throughs are provided for in a region of about 1 cm2 below the power transistor, the dissipation heat cannot be conducted by them to the cooling elements in a magnitude of over/more than 6 watts.
From EP-A-1 157 867 an electric heating device is known where the damper register and the electric control unit are likewise integrated into one structural unit. The power transistors of the control unit are arranged directly on the printed circuit board, and on their underside they are contacted by a cooling element via a bore in the printed circuit board. Such a cooling element, which is in direct contact with the power transistor, can remove the dissipation heat of the power transistor in a simple manner and in an adequate amount.
A disadvantage with this heating device is however the clamp fastening of the cooling element to the transistor. Vehicle vibrations can have a negative effect on the fastener of the cooling element so that effective heat transfer is reduced. In addition, the clamp is unable to compensate for unevenness in the contact surface of the transistor and the surface of the peg element protruding through the printed circuit board. In this way, heat transfer is impaired and cannot be calculated and the contact surface is unable to remove the dissipation heat from the power transistor equally over the entire transistor surface.
Alternatively, the transistor and the cooling element can be soldered together at the contact surface. With this fastening the clamp can be dispensed with and at the same time a reliably high heat transfer reading can be ensured at the contact surface of the transistor and the surface of the peg of the cooling element protruding through the printed circuit board. A disadvantage with this fastening is its costly production. The power transistors, due to their much smaller mass by comparison with the cooling bodies, are subject to a particular high thermal load with such a manufacturing process. The considerable mass of the cooling body requires maintaining the control circuit during the soldering process for a much longer period of time than usual at the soldering temperature in order to heat the entire cooling body to the corresponding temperature as well. In the soldering process the control circuit with the power transistors and the cooling bodies is subjected to a temperature of about 230° C. Due to its small mass, the power transistor very quickly absorbs the ambient temperature. The period of time until the cooling body is heated to a corresponding level however generally exceeds the maximum allowable time for which the power transistors may be exposed to such a temperature.